OK Problem solved (touch wood). I rang Apple support and after trying a number of possible solutions to no avail, the guy asked me if I had installed some new software around the time that this problem had started.

I then remembered installing some software called SMARTBoard, which is used with an interactive touch screen system. After installing, it caused my Macbook to behave vary strangely. When I checked with the supplier, they said service pack two of the latest release was "more or less" Leopard compatible. After pressing him further, he admitted there were a few outstanding issues and recommended I wait for the new version now in Beta. I therefore uninstalled the software by dragging it into the bin rather than use the uninstal utility (whoops).

When I checked my Library Preferences this morning, there were 4 .plist items relating to smart technologies. Deleting these seems (thus far) to have solved the problem.